(1) Field of the Invention
The present invention relates to an improvement for communicating across the air-water interface. More particularly, the invention relates to a cost-effective buoy system responsive to in-air laser beams and underwater acoustic transducers receiving and transmitting acoustic signals for bi-directional transfer of information between in-air and underwater environments.
(2) Description of the Prior Art
Effective bi-directional transfer of information between in-air and underwater platforms has been long sought since such a capability would increase the autonomy and flexibility of subsurface, surface, and air vehicles engaged in undersea warfare. However, signal transfer by contemporary communications systems has been complicated by the fact that efficient in-air signal propagation is accomplished through radio frequency (RF) transmissions while acoustic pressure waves are the most efficient means underwater. Unfortunately, RF signals do not penetrate or propagate well in water, and underwater generated acoustic signals do not readily penetrate into the air environment. Optical signals, such as laser beams can operate in both air and water environments; however, their depth range in water is limited by water clarity (signal attenuation) to typically within one hundred meters or less.
Consequently, the primary method of underwater sonar and communications relies on acoustic signal generation and propagation through the water using submerged acoustic transmission hardware.
The generation of underwater sound from an aerial platform therefore poses a challenge. Active surface ship sonar and aerial dipping sonar devices such as disclosed in U.S. Pat. No. 5,856,954 could be used for this purpose at the risk of the transmitting platform giving away its position.
Optical signals from lasers have been found to propagate well in air (depending on environmental conditions such as fog or rain) and are more covert than RF transmissions due to their confined beam width, and an opto-acoustic communication system has been developed that takes advantage of this. The opto-acoustic system (technique) provides a method for transmitting an acoustic waveform from an in-air platform into the water via conversion of optical energy at and/or slightly below the air-water interface. In the linear regime of opto-acoustics, a laser beam incident at the boundary is exponentially attenuated by the medium thus producing local temperature fluctuations that give rise to volume expansion and contraction. The volume fluctuations in turn generate a propagating pressure wave. The effect of the medium""s attenuation on the laser light is to produce an array-like structure of thermo-acoustic sources that generate modulated pressure waves at the laser amplitude and modulation frequency of the modulating laser signals. In the non-linear regime of opto-acoustics, the types of physical phenomena that are produced are based on optical energy density and intensity considerations. Broadband acoustic transients with considerable acoustic energy are created in the water. The laser pulse repetition rate can also be used to transmit selected acoustic frequencies for sonar, command and control, and communications purposes. For example, this approach can control steering of unmanned underwater vehicles (UUVs) and torpedoes.
A level of covertness and safety can be obtained using an opto-acoustic system that has been devised to remotely generate underwater acoustic signals. Sound Pressure Levels (SPL) of up to 200 dB//xcexcPa have been achieved by directing a focused, high-powered, infrared, pulsed laser beam onto the water surface. The effect of the high energy/intensity laser incident at the water""s surface is to produce a change in the phase of the water medium from water to vapor and/or plasma producing an explosive, thermo-acoustic source that generates modulated pressure waves at the laser amplitude and modulation frequency of the modulating laser signals. The remote nature of the aerial source insures that the source of the underwater acoustic transmission remains unknown to underwater platforms. Likewise, the in-air optical signal used for generating an underwater acoustic signal remains covert to in-air platforms. This method provides a means for remote, aerial generation of underwater sound, breaching the air-water interface.
However, the opto-acoustic technique requires high power pulsed lasers and focusing optics for efficient conversion of optical to thermo-acoustic energy. Also, the performance of the opto-acoustic conversion is affected by the oblique laser incidence angle at the air-water boundary, sea state roughness, and by in-water impurities.
Due to the large acoustic impedance mismatch between the air and water environments, underwater acoustic signals do not significantly penetrate into the air. Traditionally, underwater acoustic sonar requires in-water hardware for acoustic signal generation and reception. This alone makes it difficult to acoustically communicate across the air-water interface between underwater platforms such as UUVs and submarines and surface platforms from ships, unmanned aerial vehicles (UAVs), aircraft, ground based platforms and satellites. Thus, buoys were designed to receive underwater acoustic signals via underwater propagation or propagation through a direct tethered link and then reradiate the information as Radio Frequency (RF) signals into the air for subsequent detection by land or air-based platforms, see for example U.S. Pat. Nos. 6,058,071 and 5,592,156. Typically, RF signals broadcast to a large area for data reception. This is advantageous in that RF signals can be detected at great distances and relayed through satellites. However, the process is less covert and can lead to unwanted signal interception.
An alternative, the laser Doppler vibrometer (LDV) detection method had been devised to detect underwater acoustic signals by directly probing the water surface with a laser beam. This method is used for detecting acoustic signals by measuring velocity perturbations (vibrations) derived from the sound pressure at the surface of the water, and this capability may be applied to uplink communications between underwater and in-air platforms as well as aerial detection of any underwater sound for applications including marine mammal detection and tracking, and defense of surface ships from wake homing torpedoes. The LDV provides a means for covert and remote, aerial detection of underwater sound, breaching the air-water interface.
However, applying a commercial LDV involves obtaining narrow beam laser returns from the specularly reflecting water surface. Initial tests on hydrodynamic surfaces indicate that signal dropout occurs due to optical reflections arriving outside of the optical detector""s sensing area. Signal information is therefore lost intermittently and randomly, which is a detriment especially for communications applications. Irrespective of the performance of the LDV-based sensor improves with higher optical reflectivity, the air-water interface reflects only approximately 2% of the incident laser radiation and therefore limits the efficiency of this application of LDV sensors.
Thus, in accordance with this inventive concept, a need has been recognized in the state of the art for a buoyant device that enables optically controlled, bi-directional transfer of underwater sound between in-air and underwater environments that assures covert in-air operations using spatially confined, low-powered laser beams for triggering underwater transmission and for optically detecting the underwater sound.
The first object of the invention is to provide a buoy device for covertly, optically controlling, bi-directional transfer of underwater sound and optical laser signals between in-air and underwater environments.
Another object of the invention is to provide a buoy system for both translating in-air optical signals to underwater acoustic signals and translating underwater acoustic signals to optical signals transmitted through air for remote, optical reception.
Another object is to provide a cost-effective buoy to remotely generate underwater sound of known spectral content, amplitude, and phase and enhance aerial, optical detection of the underwater sound.
Another object is to provide a buoy using spatially confined, low-powered laser beams for triggering underwater transmission and optical detection of underwater sound.
Another object is to provide a buoy system using in-air laser beams and underwater acoustic transducers for bi-directional transfer of information between in-air and underwater environments.
Another object is to provide a buoy system using in-air laser beams and underwater acoustic transducers for bi-directional transfer of information between in-air and underwater environments for uplink and downlink communications and control of vehicles such as UUVs and torpedoes across the air-water interface.
Another object is to provide a buoy system using in-air laser beams and underwater acoustic transducers for bi-directional transfer of information between in-air and underwater environments that reduces laser power requirements and the difficulties associated with direct opto-acoustic conversion while maintaining in-air covert operation and remote access to the transmitting buoy.
Another object is to provide a buoy system enhancing the optical reflectivity and sensitivity for the acousto-optic (LDV-based) sensing technique while maintaining covert and remote, aerial access of underwater acoustic signal information.
Another object is to provide a buoy operating in an active mode by accepting a low-power laser beam delivering a signal through the air from a remote source to activate the buoy""s underwater acoustic transmitter.
Another object is to provide a buoy operating in the passive mode to detect underwater sound with underwater acoustic transducers and translate the detected sound into amplified vibrations that are probed by laser signals from a remote LDV sensor to allow retrieval of the detected underwater sound.
These and other objects of the invention will become more readily apparent from the ensuing specification when taken in conjunction with the appended claims.
Accordingly, the present invention is for a buoy system for bi-directional communications in-air and underwater. A hollow shell of a buoy floating on water has an upper portion in air above the surface of water and a lower portion below the surface of the water. An array of acoustic transducers is disposed in the lower portion for receiving acoustic signals and for transmitting acoustic signals through the water. A dome-shaped retro-reflective coating on the upper portion is vibrated in accordance with acoustic or other gathered information bearing data signals for retro-reflecting impinging laser illumination signals through air and conveying the acoustic and other information bearing data signals as retro-reflected data signals in air. The retro-reflective coating is controlled to vibrate in response to impinging laser control signals through air and an array of photo-detectors on the upper portion of the buoy are responsive to the impinging laser control and information signals. A control/memory/GPS module and acoustic processing-electronics section in the shell receives activation signals from the retro-reflective coating and photo-detector array and couples received acoustic signals from the transducer array and from memory as data signals to the retro-reflective coating. An array of electromechanical vibration shakers inside of and against the upper portion of the shell is driven by the optic-processing module for vibrating the retro-reflective coating, and an annular array of accelerometers is connected to the optic-processing module to monitor vibratory motion of the retro-reflective coating. Transducer elements are interspersed with the vibration shaker array under the dome-shaped retro-reflective coating. The transducer elements are connected to the optic processing module for generating signals representative of the impinging laser control and information signals. The representative generated signals from the transducer elements are coupled to the control/memory/GPS module to initiate retrieval of the information of received acoustic signals from the transducer array and memory in the control/memory/GPS module. A transmit/receive switch is connected to the control/memory/GPS module, acoustic processing-electronics section, and the transducer array to selectively enable operation of the transducer array in the passive mode and the active mode. A remote platform has at least one laser onboard for transmitting the impinging laser illumination signals and impinging laser control signals through the air. The remote platform also has a laser Doppler vibrometer-based sensor responsive to receive the data signals as the retro-reflected data signals through the air.